1. Field
This invention relates to the field of compact devices for remotely detecting the presence of biological and/or chemical materials using the reversible binding of specific molecules onto a superlattice concentrator structure in order to quickly manipulate and identify these molecules.
More particularly, it pertains to the use of a new class of solid-state concentrators of biological and/or chemical agents made from engineered Group III-V semiconductor superlattice structures, surface binding properties of which can be controlled and manipulated by optical illumination or other means, such as electrical biasing.
2. Description of the Related Art
Early detection and identification of the presence of biological agents is important because it would allow initiation of rapid evacuation of an affected area, start of immediate decontamination procedures, and provide needed information for medical treatment of personnel.
There exist a large number of known detection methods used to identify chemical and biological materials. As discussed below, all currently used methods are limited in terms of sensitivity, response time, possibility of miniaturization, or a combination thereof. The most commonly used methods are as follows:
(a) Gas and Liquid Chromatography. According to these analytical techniques, an analyzed blend is separated based on the Nernstian distribution of the components of the blend between two phases.
In the case of gas chromatography, the blend is first evaporated and then directed to a solid adsorbent where the blend is separated. In the case of liquid chromatography, a liquid adsorbent is used for separation.
After the separation, the individual components, including biological agents, are detected, usually using the differences in the components' heat transfer properties. Although chromatographical methods are quite sensitive (from the low parts per million (ppm) range for gas chromatography to the moderate ppm range for liquid chromatography), they have serious disadvantages. The response times for these methods are long and portability is very limited because it is difficult to miniaturize the equipment.
(b) Gel Electrophoresis. According to this method, particles of a gel (disperse phase) move in the dispersion medium under the influence of an electric field. The detection of an agent, including a biological and/or chemical agent, is based on the principle that the speed of the particles is proportional to the intensity of the electric field. The electric field causes the change of the electric potential on the border between the gel phase and the dispersion medium and this change is related to the nature of the gel particles.
Although the gel electrophoresis-based concentrator can be made portable, the method has a low sensitivity and very long response times leading to only limited use of the method for detection of biological and/or chemical agents.
(c) Optical Absorption and Raman Spectroscopy. The method of optical absorption is based on the transition of electrons in an excited state as a result of absorption of energy by the analyzed molecules in ultraviolet, visible and near infrared areas of the spectrum (between about 120 and 1,000 nanometers).
After such energy has been absorbed, wide-band spectra are obtained and the analyzed molecule is identified.
In Raman spectroscopy, the analyzed substance is irradiated with monochromatic radiation. When a beam of photons strikes the molecule, some photons undergo inelastic or Raman scattering and such Raman scattered photons have different frequencies and produce a spectrum of frequencies in the scattered beam that constitute the Raman spectrum of the analyzed molecule. This spectrum is used for the identification of the sample.
Even though both the optical absorption method and the Raman spectroscopy method provide a fast response time, sensitivity of both is low, and portability is limited. In addition, in the optical absorption method, multiple compounds absorb the energy, thus producing a multiplicity of the resulting spectra and obfuscation of the molecule being sought.
(d) Fluorescence Spectroscopy (Tags). According to this method, visible or ultraviolet radiation (at wavelengths between about 200 and 700 nanometers) is first absorbed by the analyzed specimen. Thereafter, the excited molecules return to the normal condition. This process is accompanied by the emission of radiation. The emission spectrum of the radiation is thus obtained and used to identify the molecule.
The fluorescence spectroscopy method has fast response time and is amenable to designing a portable instrument. However, the method has only moderate sensitivity due to background noise contamination.
(e) Upconverting Phosphor. This method provides good background rejection and has fast response times. Making a portable instrument for this method is also possible. According to this method, uniform submicron microspheres of upconverting phosphors (UCPs) are synthesized and coated with biologically active probes, such as antibodies. UCPs are materials that emit visible light upon excitation with near-infrared light. Functionalized UCP particles are used to selectively bind to captured target antigens. Visible emission following exposure to IR radiation indicates a target.
However, this method must be interfaced with pre-processing techniques, which limits the applicability and usefulness of UCPs.
(f) Mass Spectroscopy. The method of mass spectroscopy is based on the ionization of a molecule, typically by bombarding the molecule with electrons having energy between about 50 and 70 electron-volts. The process takes place in a high vacuum environment (at least 10−6 torr vacuum is required). The ions created as a result are accelerated in an electric field and separated in a magnetic field according to their mass. A mass spectrum is then created showing the mass-to-charge ratios for various ions created as a result of the fragmentation of the molecule due to the bombardment and the relative amounts of such ions. This spectrum is used to identify the original molecule.
The sensitivity of this method is high, the response time is fast and miniaturization of the instrument and portability is possible. However, the method requires expensive and fragile equipment necessary to create and maintain a very high vacuum. The method requires high power consumption and the equipment is susceptible to clutter problems.
(g) Ion Mobility Spectrometry. This method is highly sensitive, has fast response time and the instrument can be portable. Ion Mobility Spectrometry (IMS) involves the ionization of molecules and their subsequent temporal drift through an electric field. Analysis and characterization are based on separations resulting from ionic mobilities rather than ionic masses; this difference distinguishes IMS from mass spectroscopy. However, serious problems of cross-coupling of the signal with temperature and pressure limit its applicability for detection of biological and/or chemical agents.
As far as the instruments are concerned, there exist a number of current generation sensors used in detection of biological and/or chemical agents. Many of these sensors are constructed with a sensor transducer in combination with a biologically active surface.
A key element of these point detection systems involves the immobilization of the biological and/or chemical agent. This issue is very important for binding, isolating and concentrating the biological and/or chemical agent on the transducer as well as for maintaining the agent's structure, activity, and stability. Biological species of interest include various toxins, viruses, rickettsia, fungi, parasites, bacteria, or uniquely identifiable components or byproducts (DNA, RNA, proteins, sugars, etc.).
Typically, the surfaces on which biological agents are immobilized, comprise antibodies of a particular species of interest for trapping the particular biological agent (antigen). These surfaces are integrated into a transducer for sensing. Examples of such devices comprise surface acoustic wave (SAW) resonators, cantilever resonators, surface plasmon resonance reflectors, and fiber-optic based sensors that are coated with immobilizing surfaces.
The antibody-antigen (biological molecule) binding is irreversible, which both benefits these detection methods and causes complications. The limitations of irreversible binding comprise baseline drift, saturation, and contamination of the sensor surfaces. Current generation point detection systems are often dependent on time-consuming molecular amplification processes that are needed to increase the signature of the pathogen or agent.
There is a need to have compact, low cost remote concentrators of biological and/or chemical species which:    (a) are very sensitive, compact in size, and able to detect very small quantities of the compound in question;    (b) have fast response times;    (c) are portable and compact; and    (d) are more reliable and have lower false alarm rates than currently available sensors.
There exists no known prior art for compact concentrators satisfying all these enhanced requirements. Yet the need for such is acute. For the foregoing reasons, there is a necessity for a compact low-cost sensor for detection of very low amounts of biological and/or chemical substances. The present invention discloses such concentrators which can serve to bolster the performance of any existing detection method by concentrating the bio- or chemical-agent before analysis.